The present invention relates to an electronic probe and, more particularly, to an electronic probe providing a high impedance input at direct current so as not to lower the impedance of a high impedance device under test. The probe further provides a low impedance input at high frequencies in order to provide improved sensitivity without amplification.
Tri-state logic devices have a high, a low, and a high impedance mode. In the high mode or logic one mode, a tri-state logic device outputs a high voltage. In the low mode or logic zero mode, the tri-state logic device outputs a low voltage or zero volts. In the high impedance mode, the output of the tri-state logic device is neither high nor low, but rather is a high impedance output. Conventional logic devices typically have only high and low voltage modes.
Tri-state logic devices are used in many high speed communications systems because the properties of tri-state logic are adapted to accommodate multiple bus drivers. The communications are in the form of binary data consisting of high and low voltages output by the tri-state logic devices. Communications with tri-state logic devices also consist of the tri-state logic devices being in the high-impedance mode so as not to adversely affect data communications between several tri-state logic devices. For example, if the outputs of several tri-state logic are electrically connected together, only one tri-state logic device may be active. The other tri-state logic devices may be in the high impedance mode and thus will not affect the output of the active tri-state logic device.
Measuring voltages of circuits using tri-state logic devices presents many problems. For example, many tri-state logic buses have a plurality of conductors located in very small areas, which creates high concentrations of conductors in these areas. In order to measure the voltage of more than one conductor within a data bus at a time, the probes have to be very small. The probes must also not load the circuits being measured, which typically occurs when a conventional resistor divider probe is used to measure a tri-state logic circuit. For example, a conventional probe may load the output of a tri-state logic circuit that is in the high impedance mode.
Some probes use a plurality of devices in their tips in order to provide high impedance. For example, the tips may have a plurality active or passive devices located therein. These probes have many drawbacks when they are used to measure a plurality of highly concentrated conductors. Probes with a plurality of devices in their tips are bulky and may not be small enough to measure voltages on a circuit having a high concentration of conductors. Furthermore, probes with active devices or even a plurality of passive devices tend to be expensive and rather difficult to manufacture.
Therefore, a device is needed to overcome all or some of the above-described problems.
The invention is directed toward an electronic probe used to measure voltage over a broad frequency spectrum. The probe may comprise a probe tip, a transmission line, a termination portion, a filter, and an impedance device. The transmission line may have a transmission line first end and a transmission line second end, wherein the transmission line first end is connected to the probe tip. The termination portion may comprise a termination portion first end and a termination portion second end. The termination portion first end is connected to the transmission line second end and the termination portion second end is connected to a common node. The termination portion may comprise a termination resistor and a termination capacitor connected in series between the termination portion first end and the termination portion second end. The filter may comprise a filter first end and a filter second end. The filter first end is connected to the transmission line second end and the filter second end is connected to a probe output. The filter may comprise a filter resistor connected in parallel with a filter capacitor and a filter inductor connected in series with the filter resistor and filter capacitor combination. The impedance device may be connected between the probe output and the common node and may serve as a load.
The electronic probe provides high impedance for direct current voltages because the termination capacitor provides a direct current block for direct current and low frequency voltages. At higher frequencies, the impedance of the termination capacitor drops to an insignificant value. Thus, the impedance into the termination portion is substantially equivalent to the termination resistor. The termination resistor is selected to match the characteristic impedance of the transmission line, thereby reducing the incident-wave reflections.
The filter serves to offset the filtering affects inherent in the termination portion. Without the filter, the gain of the probe would vary significantly with the frequency of the measured voltage. By offsetting the filtering affects of the termination portion, the gain of the probe remains substantially constant over a broad frequency spectrum.
The probe may be electrically connected to a measurement device having an input capacitance associated therewith. The input capacitance acts as a filter and reduces the gain of the probe at high frequencies. The input capacitance also causes an impedance discontinuity that causes energy to reflect back toward the probe tip. The filter inductor serves to partially offset the effect of the input capacitance. Thus, the filter inductor offsets the capacitive discontinuity caused by the input capacitance and maintains the gain of the probe constant.